Aquaculture nets or fish-farming nets are used to raise aquatic life such as fish. The aquaculture net keeps the aquatic life controlled and contained and protects the aquatic life inside the net against predators such as sharks and sea wolfs.
The aquaculture nets are usually of the chain-link fence type. This is a fence of steel wires woven into a diamond pattern. The meshes have a dimension that is smaller than the dimension of the fish contained in the nets. Each steel wire is preformed by bending so that it exhibits a wavy pattern with maxima and minima. The maxima of a steel wire interlock with the minima of a neighbouring wire to form the patterns of a series of diamonds.
Experience has shown, however, that aquaculture nets of the chain-link fence type also have some disadvantages. Aquaculture nets have been discovered where one or more of the steel wires were broken after a limited life time.
Investigation of the failing aquaculture nets revealed that particularly the upper steel wires in the aquaculture net were broken. Indeed, the ocean or the sea forms a huge challenge with respect to corrosion-resistance. In addition to this highly corroding environment, the waves and tidings subject the aquaculture net to a continuous and repeated movement. In an aquaculture net of the chain-link fence type, each steel wire must carry the weight of the rest of the net below it. The upper wire makes point contacts with the wire just below it. These point contacts are located at a point where both the upper wire and the wire just below it are subjected to both a bending and a torsion deformation. The continuous and repeated imposed movements in this aggressive environment create fretting at the point contacts and may result in breaking the wires of the net.
Aquaculture nets with galvanized steel wires offer an acceptable resistance against bio-fouling, i.e. against fouling material that may grow on the mesh structure. Within the context of the present invention, the terms fouling material refer to fouling organisms such as barnacles, algae or molluscs, which may attach and grow to the wire material of the mesh structure. However, this fouling mechanism may be so persistent that entire openings in the meshes may be filled blocking any introduction of fresh water or nutrition into the volume inside the mesh structure.
Therefore, there is a need for aquaculture nets with better anti-fouling and anti-corrosion properties.
JP-A-2004-261023 discloses a steel wire for aquaculture nets. The steel wire has a stainless steel core and a metal coating of cupronickel: a copper nickel alloy with nickel content ranging between 10% and 30% by weight. The metal coating can be applied either by hot dipping the stainless steel core in a copper nickel bath or by plating the stainless steel core with copper, thereafter with nickel and finally applying a thermal diffusion treatment.
Copper nickel coatings have proven to provide a good resistance against corrosion because of the nickel and have proven to provide a good resistance against fouling because of the effect of copper. However, the existing prior art steel wires with copper nickel coatings lack the possibility to fine tune the thickness and the composition of the copper nickel coating because of following reasons or do not offer high quality steel wires with a thick coating.
If a hot dip copper nickel bath is used, the composition of the copper nickel coating can be varied by varying the composition of the copper nickel bath. The thickness, however, largely depends upon the wire speed of leaving the bath and upon the final degree of rolling or drawing. It is difficult, if not impossible, to obtain steel wires with a coating the thickness of which exceeds 30% of the wire diameter. Moreover, having regard to the high melting temperature of both copper and nickel in comparison with zinc, it is difficult to manufacture high-quality coated wires due to oxidation problems at the surface of the steel core.
If a thermal diffusion treatment is applied, the thickness of the copper nickel coating can be predetermined by increasing the time for (electro)-plating. It is hereby understood that the longer the plating treatment the thicker the coating and vice versa, all other parameters being constant. The composition, however, is more difficult to control. As the nickel is applied on top of the copper, nickel diffuses through the copper from the top and also from the stainless steel core since stainless steel also comprises nickel due to downstream heat treatments. This diffusion process, however, is difficult to control and to fine tune. As a result, with a thermal diffusion treatment it is impossible to obtain a coating with e.g. 90 wt % copper and 10 wt % nickel and having this same composition throughout the thickness of the coating.
The dimensions of an aquaculture net are considerable. An example of a typical dimension is 30 m×30 m×15 m, the last dimension being the depth of the net inside the water and the first two dimensions being the width and length of the net at the water surface. As a matter of example only, a net made of galvanized steel wire and of the above-mentioned dimensions has a weight above 4 metric Tonnes. An embodiment of an aquaculture net has been disclosed in W0-A1-2007/031352. As such the weight of an aquaculture net needs to be kept to a minimum. A tuneable coating guarantees thickness thus controllable weight of the wires and the net.